1. Field of the Invention
The present invention relates to a color image forming apparatus, a scanning device, a lens set, a method for fabricating a color image forming apparatus, and a storage medium for storing a program used for fabricating a color image forming apparatus.
2. Description of Related Art
Conventional color image forming apparatuses form color images by superposing toner images in the colors cyan (C), magenta (M), yellow (Y), and black (K). Color image forming apparatuses that employ the tandem system are provided with a plurality of image bearing members (photosensitive drums or the like), one for each color. The surface of each image bearing member is exposed by a light beam according to image data corresponding to each color image, forming a latent image that is subsequently developed with toner. The toner images of each color are then transferred sequentially and superposed over one another on an intermediate transfer medium or a paper or other recording medium, thereby forming a color image.
The laser printer is one conventional example of an image forming apparatus that scans and exposes an image bearing member. In this exposure process, a light-emitting unit of the laser printer, such as a semiconductor laser, emits a laser beam. When laser beam, having divergent rays, passes through a collimator lens, the collimator lens converts the light beam into substantially parallel rays. In order to perform face tangle error correction, a cylindrical lens is provided to converge the light rays in a sub-scanning direction, guiding the light onto a polygon mirror that is rotating at a high speed. The light beam is reflected off the polygon mirror, passes through a scanning device, including an fθ lens, a cylindrical lens, and other scanning lenses, and is scanned over the surface of the image bearing member.
The fθ lens functions to focus the light beam and simultaneously correct the beam so that the beam spot is scanned over the image bearing member at a constant velocity Recently, nonspherical lenses formed by resin molds have been widely used in scanning units having an fθ lens. A color image forming apparatus is provided with a semiconductor laser for forming images in each color. The light beam irradiated from the semiconductor laser is deflected by a polygon mirror and guided onto the surface of the image bearing member by a scanning unit provided for each color (although some lenses are shared among units), scanning the laser beam over the image bearing member.
However, when color images are formed by superposing toner images in each color, problems in registration among colors can occur. For example, as disclosed in Japanese patent-application publication No. 2003-262813, color misregistration occurs when the position of the laser spot determined by each scanning unit for forming images on the image bearing member deviates relative to the laser spots of other colors. Specifically, if the slope, curvature, and the like of scan lines for each scanning unit do not align, color registration may be off, resulting in a decline in image quality. Although curves and the like in scan lines may occur due to various reasons, one such reason is a difference in cavities, that is, error in precision due to the cavities when lenses are molded with a plurality of cavities. Japanese patent-application publication No. 2003-262813 describes a technology designed to reduce the error in precision.
In the technology disclosed in Japanese patent-application publication No. 2003-262813, cavities are sorted into groups based on variations in optical characteristics caused by differences among cavities so that a relative difference in optical characteristic values among cavities in the same group fall within a tolerable range. One lens for each scanning unit is selected from a single group in order to suppress deviations in color registration.
When a scanning unit is provided for each of four colors CMYK, lenses constituting the scanning units for the two colors cyan and magenta, for example, may each include a lens molded with cavities having the same specifications. In such a case, obviously it is desirable to use two lenses molded using the same cavity in order to eliminate the problem of differences between cavities. However, this is not always possible, particularly in mass-production. Hence, the technology in Japanese patent-application publication No. 2003-262813 attempts to reduce misregistration by sorting the cavities into groups of lenses for suppressing problems in registration among colors as much as possible.